The present disclosure relates to a display unit performing picture display using, for example, a laser beam, and a polarization splitting multiplexing device and an optical system that are applied to such a display unit.
In recent years, as a light source of an illumination optical system in a projector (a projection type display unit), a laser attracts attention instead of a light emitting diode (LED). This is because using the laser as a light source expands color reproduction range, and contributes to achievement of low power consumption.
However, the laser is coherent light, and therefore if the laser is irradiated to a diffusing surface, spotty interference pattern is likely to be observed. Such interference pattern is called speckle pattern that is caused by the fact that light diffused by the diffusing surface interferes due to microscopic irregularity of the diffusing surface, resulting in decline in picture quality. Therefore, it is desirable to make the speckle pattern as less visible as possible.
To solve such a disadvantage, for example, there has been reported a method in which one laser beam is split into two polarized beams, and then multiplexing is performed so that one polarized beam has optical path delay with respect to the other polarized beam to reduce speckle (for example, see PTL 1 and PTL 2). In PTL 1, there is reported a method in which a laser beam is split into a P-polarized light beam and an S-polarized light beam by a first polarization beam splitter, and then the S-polarized light beam is multiplexed with the P-polarized light beam by a second polarization beam splitter through a folding prism. In addition, in PTL 2, there is reported a method in which a laser beam is split into a P-polarized light beam and an S-polarized light beam by a polarization beam splitter, and then these polarized light beams are reflected by a mirror toward the polarization beam splitter, and further the S-polarized light beam and the P-polarized light beam are multiplexed by a quarter-wavelength plate provided between the polarization beam splitter and the mirror. In PTL 1 and PTL 2, speckle is reduced by setting an optical delay distance between the P-polarized light beam and the S-polarized light beam to a coherence length or more. In addition, there is reported a method of splitting a laser beam with use of a half mirror (for example, see PTL 3).